Hydraulic control device for motor vehicle clutch in particular

ABSTRACT

A clutch control device ( 10 ) comprising a cylinder body ( 17 ) and a piston ( 21, 42 ) axially sliding in an inner bore ( 22 ) of the cylinder body ( 17 ) and at least a dynamic seal ( 59 A,  59 C) borne by the cylinder body ( 17, 17 B). The cylinder body ( 17 ) comprises, axially arranged successively from the front rearwards, a blind front component ( 17 A) comprising a base ( 24 ), a central component ( 17 B) and a rear component ( 17 C) open rearwards made of plastic material and assembled together sealed by welding ( 58 A,  58 C) with indirect heat input, and the inner bore ( 22 ) comprises a front portion ( 22 A) and a rear portion ( 22 C), and the dynamic seal ( 59 A,  59 C) is housed in the central component ( 17   b ) of the cylinder body ( 17 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for hydraulic brake or clutchcontrol, in particular for a motor vehicle, comprising at least oneemitter or receiver cylinder having a piston.

More particularly, the invention relates to apparatus for hydraulicbrake or clutch control for a motor vehicle, comprising at least onecylinder which includes a cylinder body having at the front a baseportion and being open at the rear, a piston sliding axially in aninternal bore of the cylinder body, and at least one dynamic sealingmember carried by the cylinder body for cooperation with the outerperiphery of the piston, wherein the cylinder body is of syntheticmaterial such as plastics material, and wherein the piston has a frontface bounding a hydraulic chamber.

2. Description of Related Art

An apparatus of that kind is described, for example, in the documentsDE-U-29516488, FR-B-274920 or in French patent application No. 98 07213of Jun. 9, 1998.

SUMMARY OF THE INVENTION

An object of the invention is to propose a control apparatus of the typementioned above, having reduced manufacturing costs and simplifying thedesign of the cylinder body.

With this in view, the control apparatus in accordance with theinvention is characterised in that the cylinder body includes, arrangedconsecutively in the axial direction from front to rear, a blind frontpiece which includes the said base portion, a central piece, and a rearpiece which is open towards the rear, the said pieces being joinedtogether in a sealed manner, in that the internal bore consists of afront portion and a rear portion, which are formed in the front pieceand rear piece respectively of the cylinder body, and in that the saiddynamic sealing member is mounted in the central piece of the cylinderbody.

With such a design it is in particular possible to standardise the frontand rear pieces of the cylinder body and to create a range of productsby making a range of central pieces which are different from each other,in particular for the purpose of incorporating means for feeding thehydraulic chamber and means for fastening the control apparatus, whichvary from one apparatus to another, especially as a function of themodel and type of vehicle that it is to equip.

In accordance with further features of the invention:

the said sealed joint is an adhesively bonded joint;

the said sealed joint is a joint which is made by welding with indirectapplication of heat;

the said sealed joint is a joint which is made by welding with the aidof at least one energy source of the laser type;

the said sealed joint is a welded joint made by infrared radiation, forexample of the laser type;

the said sealed joint is a joint made by ultrasonic welding;

the said sealed joint is a joint made by friction welding; the saidsealed joint is a joint made by high frequency welding;

the said sealed joint is a welded joint made by induction welding offerromagnetic members;

the sealed joint is obtained by mirror welding;

the piston extends through the central piece of the cylinder body with aradial clearance;

an axially oriented tubular rear portion and an axially oriented tubularfront portion of the front piece and rear piece of the cylinder body,respectively, are welded to a complementary front portion and rearportion, respectively, of the central piece of the cylinder body;

the outer peripheral surface of the said tubular portion is fixedsealingly, by welding with indirect application of heat, to an internalperipheral surface of the said complementary portion of the centralpiece of the cylinder body;

the inner peripheral surface of the said tubular portion is fixedsealingly, by welding with indirect application of heat, to an externalperipheral surface of the said complementary portion of the centralpiece of the cylinder body;

the said tubular portion is in axial abutment against a radial shoulderof the central piece of the cylinder body;

the central piece includes a transverse duct which is open in asubstantially radial direction into an axially oriented central hole ofthe central piece in facing relationship with the outer periphery of thepiston;

the central piece of the cylinder body is configured as a transverseplate for fastening the control apparatus;

the apparatus includes two dynamic sealing members, namely a frontmember and a rear member, which are mounted within the central piece ofthe cylinder body;

the front dynamic sealing member and the rear dynamic sealing member areeach, respectively, disposed in a housing formed in the central piece ofthe cylinder body, which is open radially inwards;

the rear end of the rear piece of the cylinder body includes at leastone abutment adapted to cooperate with a rear end of the piston so as tolimit rearward axial displacement of the piston;

the piston includes a peripheral piston jacket which is fixed axially tothe piston and which slides axially in the internal bore of the cylinderbody;

the piston jacket has at least one hole for providing communicationbetween the inside and the outside of the piston jacket, and the hole isadapted to be displaced to either side of a dynamic sealing member;

the piston jacket is of metal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will appear on areading of the following detailed description, for an understanding ofwhich, reference will be made to the attached drawings in which:

FIG. 1 is a view in longitudinal axial cross section of a firstembodiment of an emitter cylinder made in accordance with the mainfeature of the invention, and in which the piston is shown in aretracted position;

FIG. 2 is a detail view, on a larger scale, of the central part of theemitter cylinder of FIG. 1, showing the sealed fastening of the threecomponents of the cylinder body; and

FIG. 3 is a view similar to that in FIG. 2, but shows a secondembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, those elements which are identical,similar or analagous to each other will be designated by the samereference signs.

The drawings show a control cylinder in the form of an emitter in acontrol apparatus for a motor vehicle clutch.

Such an apparatus (not shown) includes an emitter cylinder which isconnected through a duct to a receiver cylinder, the structure of whichis similar to the emitter cylinder.

Each receiver or emitter cylinder comprises a piston which is movableaxially inside a cylinder body so as to define a variable volumehydraulic chamber. A connecting port, into which the duct exhausts, isopen into the hydraulic chamber.

The emitter cylinder includes a piston rod which is coupled for exampleto a clutch pedal operated by the driver, or to an actuator which isstarted in accordance with predetermined programmes.

The piston of the emitter cylinder is arranged to expel a fluid such asoil, which is contained in the hydraulic chamber, towards the channel orduct connecting the emitter cylinder to the receiver cylinder.

When the clutch is engaged, the volume of the hydraulic chamber of theemitter cylinder is at a maximum, while the volume of the controlchamber of the receiver cylinder is at a minimum.

In the operation of disengaging the clutch, the volume of the hydraulicchamber of the emitter cylinder diminishes, while the volume of thecontrol chamber of the receiver cylinder increases.

The piston of the receiver cylinder then works on a rod which acts forexample on the declutching fork that actuates the clutch release bearingwhich is part of the friction clutch.

In another version, the piston works in traction on the said forkthrough a cable or other means.

Each of the emitter and receiver cylinders includes a spring which worksbetween the piston and the base of the cylinder body. When the driverreleases pressure on the clutch pedal, or when the actuator returns toits initial clutch-engaged position, the return spring of the emittercylinder expands so as to return the piston to its initial position,while the return spring of the emitter cylinder is compressed by theclutch spring, such as a diaphragm, which thereby returns the piston ofthe receiver cylinder to its initial position.

In the drawings, the emitter cylinder 10 is shown in a delivery positionand comprises a cylinder body 17 which has an inlet port 18, closed offby a plug 19. It is through this inlet port that the duct or channel(not shown) which connects the receiver cylinder (not shown) to theemitter cylinder 10, is open into the hydraulic chamber 20 of theemitter cylinder.

The chamber 20 has a variable volume, and it is bounded by the body 17and the piston 21 which is mounted for axial movement within thecylinder body 17.

The cylinder body 17 is of generally tubular form, and in accordancewith the features of the invention it consists of three distinct pieces,which are made separately by moulding in plastics materials, and whichare disposed in axial succession from front to rear, that is to say fromleft to right in the drawings, and which consist of a front piece 17Awhich includes the orifice 18 and the plug 19 at its front axial end, acentral or intermediate piece 17B, and a rear piece 17C.

The cylinder body 17, in its three pieces 17A, 17B and 17C, has aninternal bore 22 which is generally blind and formed on an axis X-Xcorresponding to the general axis of symmetry of the cylinder body 17,with, at the front, a transversely oriented base portion 24 in which theinlet port 18 is open, and which is part of a connecting portion 25 forconnection of the above mentioned duct, for example by seaming, theconnecting portion 25 having for this purpose internal passages 26 forreceiving a clip which fastens the duct to the emitter cylinder. Inanother version, the connecting portion may be of the screw type.

The cylinder body 17 is generally open at the rear, that is to say itsrear piece 17C is itself open towards the rear.

Each of the two axial end pieces, that is to say the front piece 17A andthe rear piece 17B, is in the general form of a tubular member ofgenerally simple design, easy to make by moulding in plastics material.

The internal bore 22, for guiding the piston 21 in sliding movement,comprises, in accordance with the features of the invention, a frontportion 22A formed in the front piece 17A, and a rear portion 22C whichis formed in the rear piece 17C, the two portions, namely the frontportion 22A and rear portion 22C, being aligned and coaxial with eachother.

The piston 21 is arranged to slide in the cylinder body 17, and moreprecisely in the internal bore 22 of the latter, forward from the rear,starting from the retracted or declutched position shown in FIG. 1,towards an advanced or clutch engaged position (not shown).

In the retracted or declutched position shown in FIG. 1, the hydraulicchamber 20 and the control fluid are pressurised, while in the advancedposition, the chamber 20 and the control fluid are depressurised.

A generally frusto-conical projection 28 is formed integrally bymoulding with the transverse base portion 24, and it extends axiallytowards the rear within the bore 22, so as to constitute a centre forcentring a return spring 29, which in this example is a helical springacting between the cylinder body 17 and the piston 21, and moreprecisely between the transverse base portion 24 and the front of thepiston 21.

The port 18 is of course formed through the base portion 24, and is openat the side of the projection 28 which is formed integrally by mouldingwith the base portion 24.

The body of the piston 21 is a component moulded in plastics material ina generally tubular form, and it has in its front part a transverse wall30, the front transverse face 31 of which bounds the hydraulic chamber20 axially at the rear, while its rear face 32 is in cooperation withthe head of a piston rod 33.

The body of the piston 21 is extended axially forward from its fronttransverse face 30 by a chimney portion 34 which has a frusto-conicalexternal profile, with a blind central hole 35 of cylindrical form, theaxially oriented base of which is defined by the front face 31 of thewall 30.

The helical return spring 29 is mounted around the chimney portion 34,the base of which serves for centring the return spring 29, and the rearaxial end of the spring bears on the front face 31 of the transversewall 30 of the piston 21.

The spring 29 therefore works axially between the base portion 24 andthe front face 31.

The rear axial transverse end face 36 of the projection 28 mayconstitute an abutment with which the transverse front axial end face 37of the chimney portion 34 is arranged to cooperate.

More precisely, in the deployed position the piston 21 comes intocontact with the face 36 of the projection 28 through the front face 37of the chimney portion 34, the cylinder body 17 accordingly having anabutment 28 which limits the axial forward displacement of the piston21.

The tubular body of the piston 21 of plastics material is extendedaxially towards the rear from the transverse wall 30 by a sleeve portion38 which has a bore 39, the front base portion of which is defined bythe rear face 32 of the transverse wall 30 of the piston 21.

The base 32 of the bore 39 is of generally hemispherical form, forreceiving the complementary head 40 formed at the axial front end of thepiston rod 33.

The head 40, and therefore the piston rod 33, is retained axially withinthe bore 39, and is therefore held with respect to the piston 21, by anelastically deformable element 26 which will not be described in anygreater detail here.

The rod 33 is accordingly mounted for rotation within the piston 21, andmore precisely with respect to the base 32 of the sleeve portion 38.

The piston 21 carries at its cylindrical outer periphery a static seal41 for sealing the piston, which in this example is an O-ring sealmounted in an internal radial groove formed in the cylindrical outerperiphery in line with the transverse wall 30. Other forms of staticseals may be envisaged.

The static seal 41 is arranged to cooperate with the cylindrical innerperipheral surface of a piston jacket 42 which in this example is ofmetal.

The piston jacket 42 is a tubular element which surrounds the piston 21,the latter being received within the jacket 42.

The piston jacket 42 is made for example of anodised aluminium ortreated steel. The piston jacket 42 extends forward, in axial projectionwith respect to the chimney portion 34, in such a way that, in theadvanced position which is not shown, the chimney portion 34 is incontact with the projection 28, while the transverse front terminal edge42A is spaced axially away from the transverse base portion 24 thatfaces it. The dimensions of the piston jacket 42 are such that thechimney portion 34 and the return spring 29 are received radially, witha clearance, within the piston jacket 42.

The piston jacket 42 is in intimate contact on its cylindrical innerperiphery with the cylindrical outer periphery of the piston 21 and withthe outer periphery of the static seal 41.

The piston jacket 42 is fixed axially with respect to the piston 21,this being achieved firstly by a radial gripping effect between thepiston 21 and the piston jacket 42, with cooperation of the static seal41. In order to complete this relative axial immobilisation of thepiston 21 with respect to the piston jacket 42, and their immobilisationin rotation, it is in this example proposed that the piston jacket 42has, in the region of its rear axial terminal edge 42C, lugs 44 whichare received in engagement within a fastening groove 43 which is formedradially, facing towards the axis X-X, in the rear end portion 38C ofthe sleeve portion 38 of the piston 21.

Other means for coupling the piston 21 to the piston jacket 42, axiallyand in rotation, may be envisaged without departing from the scope ofthe invention.

In all cases, there is provided in this way a unitary sub-assemblyconsisting of the piston 21 and piston jacket 42, constituting a unitwhich is movable axially in the internal bore 22 of the cylinder body17, the piston jacket 42 and the piston 21 being components which areconcentric and coaxial with the axis X-X.

The cylinder body is arranged to be fixed on a fixed part of thevehicle, and is therefore part of the generally fixed portion of theemitter cylinder 10, while the piston 21 with its piston rod 33 is partof the movable portion of the emitter cylinder.

In accordance with the features of the invention, the central portion17B of the cylinder body 17 is made in the form of an intermediate platewhich is disposed axially between the front portion 17A and the rearportion 17C of the body 17.

The plate 17B extends transversely, in a radially oriented plane atright angles to the axis X-X.

The central piece 17B constitutes a fastening plate and is moulded inplastics material, and it may for example have fastening ears 46 whichinclude holes 47 for the passage through them of fastening members,usually screws, for securing the cylinder body 17 and therefore theemitter cylinder 10, on a fixed part of the motor vehicle.

FIG. 1 also shows a tube 49 fixed to the central piece 17B of thecylinder body 17.

The tube 49 enables the interior of the cylinder body 17 to be connectedto a main feedback reservoir (not shown), which is mounted outside theemitter cylinder 10, the hydraulic chambers of the emitter and receivercylinders being filled with a hydraulic fluid, such as oil for example.

The reservoir is partly filled by the hydraulic fluid and it isconnected through a duct, not shown in the drawings, to the tube 49,with the latter opening into an aperture 50 which is formed in thecentral piece 17B in its transverse front end face 45A. The aperture 50is one end of a duct 51 which is oriented at an inclination to the axisX-X, and which extends through the central piece 17B from its transversefront face 45A, to open towards the rear and axially inwards into acylindrical central hole 53, oriented axially and formed in the solidcentral portion 54, which is of generally cylindrical annular form, ofthe central piece 17B, and which, as will be explained in detail laterherein, is substantially coaxial and concentric with the front portion22A and rear portion 22C of the internal bore 22, the internal diameterof the cylindrical hole 53 being in this example greater than theinternal diameter of the bore 22, so that the cylindrical outerperipheral surface of the piston jacket 42 extends through the hole 53with a radial clearance between these two elements.

The tube 49 is fastened to the central piece 17B by force-fitting of itsrear end in a complementary housing formed in the transverse front face45A of the central piece 17B, and then for example by a weldingoperation with indirect heating.

With this in view, the tube 49 is of course made in a plastics materialwhich is compatible with that in which the central piece 17B of thecylinder body 17 is moulded.

The first embodiment, according to the invention, of the means forjoining and sealingly fastening the three components which constitutethe cylinder body 17 together, will now be described with reference toFIGS. 1 and 2.

In the first embodiment shown in FIGS. 1 and 2, the axial rear endportion 56A of the front piece 17A, and the axial front end portion 56Cof the rear piece 17C, are each formed as a solid tubular portion, theradial thickness of which is slightly greater than that of the tubularmain portion of the front piece 17A or rear piece 17C respectively, thetwo tubular portions 56A and 56C preferably having the same radialdimensions, that is to say, in particular, the same outer and innerdiameters.

Each tubular portion 56A, 56C is received axially in a complementaryhousing formed in the central piece 17B in facing relationship with it.

More precisely, the solid central portion 54 of the central piece 17B inthe form of a plate includes two sleeve portions or bushes, namely afront sleeve portion 27A and a rear sleeve portion 27B, which extendaxially forwards and rearwards respectively from the opposed fronttransverse face 45A and rear transverse face 45C respectively of thecentral piece 17B.

Each bush 27A, 27C is of generally cylindrical annular form and isformed integrally by moulding with the solid central portion 54 of thecentral piece 17B, and it defines an internal cylindrical peripheralsurface 23A, 23C, which are open axially towards the front and rearrespectively, and which are bounded axially towards the rear and thefront respectively by a transverse front shoulder 64A and a transverserear shoulder 64C respectively, each of which constitutes an axialabutment which determines the relative axial position of the front andrear pieces 17A and 17C respectively with respect to the central piece17B, because of the abutting engagement against the said shoulder 64A,64C of the respective transverse end face, namely the rear face 156A ofthe rear tubular portion 56A and the front face 156C of the tubularfront portion 56C.

The cylindrical housings 23A and 23C thus precisely determine therelative axial position of the front piece 17A and the rear piece 17Brespectively of the cylinder body 17, and also the alignment and perfectcoaxiality of the front portion 22A and rear portion 22C of the internalbore 22 of the cylinder body 17 in which the piston jacket 42 is mountedfor sliding movement.

Each tubular portion 56A, 56C is force-fitted axially in thecorresponding sleeve portion 27A, 27C until the faces 156A, 156C comeinto axial engagement with the shoulders 64A and 64C.

The sealed fastening of the front piece 17A and rear piece 17C on thecentral piece 17B is, in accordance with the features of the invention,achieved by welding the tubular portions 56A, 56C in the housings 23A,23C with indirect application of heat.

Thanks to the design according to the invention, the sealed welding zone58A, 58C corresponds to the interface between the cylindrical outerperipheral surfaces of the tubular portions 56A, 56C and the cylindricalinner peripheral surfaces 23A, 23C of the sleeve portions 27A, 27C.

The welding zone 58A, 58C, in which the operation of welding by indirectheating is effected, is thus very precise and is limited without thewelding operation having any detrimental impact on the other parts andother components of the emitter 10.

The design in accordance with the embodiment shown in FIGS. 1 and 2 ismore particularly well adapted to achieving an assembly with sealedjoints, by the use of a welding operation with infrared radiation.

To this end, the central piece 17B according to the invention is made bymoulding in a plastics material which is transparent to infrared rays,while the front end piece 17A and rear end piece 17B are preferably madein a plastics material that absorbs infrared rays, for example thetransparent plastics material identical to the piece 17B, and to whichis added at least one additive such as carbon in a proportion of 1 to 2%so as to make it absorbent to the said infrared rays.

By way of variant, and making use of twin-material moulding methods,only the parts which are arranged to be welded together are made in thematerials transparent or absorbent to infrared rays.

The infrared welding operation is then for example carried out using alaser beam which causes the interface zone 58A, 58C to be welded.

The invention is not limited to the operation of making a sealed weld byinfrared radiation.

It is also possible to achieve this sealed fastening by ultrasonicwelding, high frequency welding, or induction welding to form jointsbetween ferromagnetic members, with the joints being able in particularto be formed by introducing small ferromagnetic particles directly intothe components to be welded together.

In another version, the joint is made by mirror welding, that is to saywelding with the aid of an intermediate heating piece which is withdrawnafterwards.

The effectiveness with which the hydraulic chamber 20 is sealed withrespect to the outside is in this example obtained by means of twodynamic seals, namely a front seal 59A and a rear seal 59C, which inaccordance with one feature of the invention are carried by the centralpiece 17B of the cylinder body 17, and each of which cooperates with theouter periphery of the piston jacket 42, that is to say with thecylindrical outer surface of the said piston jacket 42.

Each dynamic seal, namely the front seal 59A and rear seal 59C, is inthis example a lipped channel seal which is mounted in a complementaryseating 123A, 123C formed in the solid central portion 54 of the centralpiece 17B, and open radially inwards so that each dynamic seal 59A, 59Cis able to cooperate through its radially inner lip with the outercylindrical surface of the piston jacket 42. In another version, eachdynamic seal may be a composite sealing member.

The central piece 17B thus serves, by virtue of its solid centralportion 54, as a seal box which, in this example, carries two dynamicsealing members, though a single dynamic sealing member may be providedwithout departing from the scope of the invention.

The seating 123A is open axially at the front, in such a way that thefront dynamic sealing member 59A is in axial facing relationship withthe rear transverse face 156A of the tubular portion 56A.

The piston jacket 42 has at least one hole 62 for putting the interiorand exterior of the piston jacket 42 into communication with each other.

The hole 62 is located, as a function of the axial movement of thepiston 21 with its piston jacket 42, on either side of the lip of thefront dynamic sealing member 59A. When the clutch is engaged, the hole62 provides communication between the hydraulic control chamber 20 andthe external main reservoir. During actuation of the clutch, thecommunication is interrupted because the hole 62 is displaced withrespect to the seal 59A, forwards with reference to FIG. 2.

Several holes 62 may of course be provided in a common transverse plane,so as to give a larger flow of fluid and to avoid the need to adopt anangular orientation during fitting, the holes 62 being spaced apartcircumferentially at regular intervals.

Communication between the hydraulic chamber 20 and the reservoir takesplace through the holes 62 and the annular axial space delimited betweenthe external cylindrical surface of the piston jacket 42 and theinternal cylindrical surface of the hole 53 of the solid central portion54 of the central piece 17B.

In accordance with an aspect which is not shown in the drawings, and soas to prevent fluid from escaping to the outside, a sealing bellows maybe provided, interposed between the cylinder body 17 and the piston rod33, for example between the central piece 17B and the piston rod.

The presence of two dynamic sealing members 59A, 59C can however enablethe need for a complementary sealing bellows to be avoided.

In that case a complementary cap (not shown) can if necessary beprovided for protecting the rear piece 17C, which is for example fixedaround the rear sleeve portion 27C of the central piece 17B, and whichsurrounds the rear piece 17C, with the piston rod 33 passing through it.

Such a protective cap can of course surround a sealing bellows.

The second embodiment shown in detail in FIG. 3 differs from theforegoing as regards the arrangement of the tubular portions and thecomplementary sleeve portions, so that it proposes an embodiment whichis more particularly adapted for a friction welding operation to achievea sealed fastening.

To this end, the front sleeve portion 27A and rear portion 27C arereceived axially within complementary portions of the tubular portions56A, 56C which are arranged radially on the outside of the sleeveportions 27A, 27C, that is to say each of the two sealed welding zones58A, 58C corresponds to the interface zone between the externalcylindrical surfaces of the sleeve portions 27A, 27C and thecomplementary internal cylindrical surfaces of the external radial partsof the tubular portions 56A, 56C, which are formed accordingly.

The sealed weld in this example is preferably made by friction weldingin the region of the zones 58A and 58C. Welding is carried outexternally with respect to the sleeve portions 27A, 27C by vibration andrelative rotation of the front piece 17A and rear piece 17C with respectto the central piece 17B.

In another version not shown, and without departing from the scope ofthe invention, one of the outer axial pieces 17A or 17C can be designedas in the first embodiment, while the other outer axial piece 17C or 17Ais designed as in the second embodiment.

The two outer axial pieces 17A, 17C may be made by moulding in the sameplastics material or in a different plastics material, depending, inparticular, on what technique is used for making the sealed weld joiningeach outer axial piece to the central piece 17B.

Without departing from the scope of the invention it is also possible todesign the three pieces in mechanical inversion, that is to say thecentral piece 17B includes tubular portions which extend axiallyoutwards and which are received in complementary sleeve portions of thetwo outer axial pieces 17A and 17C.

In another version, the joint is made by adhesive bonding. It is ofcourse possible to fix one of the pieces 17A, 17C by adhesive bonding onthe central piece 17B and to secure the other one of the pieces 17A, 17Cby welding on the piece 17B. Thanks to the invention it is possible tofix in position, for example, the pieces 17A, 17B in one place ofmanufacture, and then to secure the piece 17C finally in anothermanufacturing location. In all cases, the central piece constitutes aseal carrier. The presence of the spring 29 is of course not essential,as the return function may be ensured by means of a spring associatedwith the brake or clutch pedal. The piston may be made entirely ofplastics material, but metallised using surface treatment for makindcontact with the seals. Friction welding is to be understood to mean,for example, vibration welding, rotation welding, or mirror welding.Preferably the pieces 17A, 17B are force-fitted into the piece 17C so asto enable the weld or adhesive bond to be optimised. It is of coursepossible to carry out a conical force-fit. During the welding operation,heat may be applied directly to at least one of the pieces to be welded.

What is claimed is:
 1. Apparatus (10) for hydraulic brake or clutchcontrol, comprising at least one cylinder which includes a cylinder body(17) having at the front a base portion (24) and being open at the rear,a piston (21, 42) sliding axially in an internal bore (22) of thecylinder body (17), and at least one dynamic sealing member (59A, 59C)carried by the cylinder body (17, 17B) for cooperation with an outerperiphery (42) of the piston (21), wherein the cylinder body (17) is ofsynthetic material, and wherein the piston (21) has a front face (31)bounding a hydraulic chamber (20), wherein the cylinder body (17)includes, arranged consecutively in the axial direction from front torear, a blind front piece (17A) which includes said base portion (24), acentral piece (17B), and a rear piece (17C) which is open towards therear, said front, central and rear pieces being joined together in asealed manner, wherein the internal bore (22) consists of a frontportion (22A) and a rear portion (22C), which are formed in the frontpiece (17A) and rear piece (17C) respectively of the cylinder body (17),said piston extending within said internal bore (22) from said frontportion (22A) to said rear portion (22C), and wherein said dynamicsealing member (59A, 59C) is mounted in the central piece (17B) of thecylinder body (17).
 2. Control apparatus according to claim 1, whereinsaid sealed joint is obtained by adhesive bonding.
 3. Control apparatusaccording to claim 1, wherein said joint is obtained by welding withindirect application of heat.
 4. Control apparatus according to claim 1,wherein the sealed joint is obtained by mirror welding.
 5. Controlapparatus according to claim 3, wherein said sealed joint (58A, 58C) isa welded joint made by infrared radiation.
 6. Control apparatusaccording to claim 3, wherein said sealed joint (58A, 58C) is a jointmade by ultrasonic welding.
 7. Control apparatus according to claim 3,wherein said sealed joint (58A, 58C) is a joint made by frictionwelding.
 8. Control apparatus according to claim 3, wherein said sealedjoint (58A, 58C) is a joint made by high frequency welding.
 9. Controlapparatus according to claim 3, wherein said sealed joint (58A, 58C) isa welded joint made by induction welding of ferromagnetic members. 10.Control apparatus according to claim 1, wherein the piston (21, 42)extends through the central piece (17B) of the cylinder body (17) with aradial clearance.
 11. Control apparatus according to claim 1, wherein anaxially oriented tubular rear portion (56A) and an axially orientedtubular front portion (56C) of the front piece (17A) and rear piece(17C) of the cylinder body (17), respectively, are welded to acomplementary front portion (27A) and rear portion, respectively, of thecentral piece (17B) of the cylinder body (17).
 12. Control apparatusaccording to claim 3, wherein the outer peripheral surface of saidtubular portion (56A, 56C) is fixed sealingly, by welding with indirectapplication of heat, to an internal peripheral surface (23A, 23C) ofsaid complementary portion (27A, 27C) of the central piece (17B) of thecylinder body (17).
 13. Control apparatus according to claim 3, whereinthe inner peripheral surface of said tubular portion (56A, 56C) is fixedsealingly, by welding with indirect application of heat, to an externalperipheral surface of said complementary portion (27A, 27C) of thecentral piece (17B) of the cylinder body (17).
 14. Control apparatusaccording to claim 13, wherein said tubular portion (56A, 56C) is inaxial abutment against a radial shoulder (64A, 64C) of the central piece(17B) of the cylinder body (17).
 15. Control apparatus according toclaim 1, wherein the central piece (17B) includes a transverse duct (51)which is open in a substantially radial direction into an axiallyoriented central hole (53) of the central piece (17B, 54) in facingrelationship with the outer periphery (42) of the piston (21). 16.Control apparatus according to claim 1, wherein the central piece (17B)of the cylinder body (17) is configured as a transverse plate forfastening the control apparatus (10).
 17. Control apparatus according toclaim 1, further comprising a front dynamic sealing member and a reardynamic sealing member (59A, 59C), which are mounted within the centralpiece (17B) of the cylinder body.
 18. Control apparatus according toclaim 17, wherein the front dynamic sealing member (59A) and the reardynamic sealing member (59C) are each, respectively, disposed in ahousing (123A, 123B) formed in the central piece (17B) of the cylinderbody, which is open radially inwards.
 19. Control apparatus according toclaim 1, wherein the rear end of the rear piece (17C) of the cylinderbody includes at least one abutment adapted to cooperate with a rear endof the piston so as to limit rearward axial displacement of the piston(21, 42).
 20. Control apparatus according to claim 1, wherein the piston(21) includes a peripheral piston jacket (42) which is fixed axially tothe piston and which slides axially in the internal bore (22, 22A, 22C)of the cylinder body (17).
 21. Control apparatus according to claim 20,wherein the piston jacket (42) has at least one hole (62) for providingcommunication between the inside and the outside of the piston jacket(42), and wherein the hole (62) is adapted to be displaced to eitherside of a dynamic sealing member (59A).